The speed and capability of electronic appliances continues to increase proportionally with the reduction in their sizes. Cellular phones, for example, continue to supply ever more utility in an ever smaller form factor. The “real estate” inside such equipment gets more crowded and, not surprisingly, more valuable. As a result, ways have to be found to reduce chip footprint in an electronic device.
One way to reduce footprint size is to reduce the number of connections of an individual semiconductor chip. However, the increasing complexity desired by manufacturers and designers works against reducing the input and output connections of a semiconductor chip.
Another way of limiting the number of required connections in a chip, without reducing the chip's capabilities and complexities, is to put more than one function on a given connection pin. One example of this is putting a seldom-used command input on the same pin as a data input. One implementation could be putting a re-program enable (RPE) for an EEPROM device on the same pin as a data line. The device itself can distinguish between data and RPE inputs because data inputs are between ground (0.0 Volts) and supply voltage but RPE inputs are higher, on the order of 12 volts. Supply voltage in modern devices can be as low as 1.8 volts, or even lower.
However, a device configured to operate in a low voltage environment, such as 1.8 volts, is likely to be damaged when hit with an input of 12 volts, such as is commonly used for re-program enables, or other special command inputs. Command inputs or other special functions, it is noted, cannot be handled by electro-static discharge protections because they need to be processed as commands.
Accordingly, there is a need for accommodating a multifunction input to a semiconductor chip that is tolerant to a high voltage input on the same input pad that responds to logic level voltages. The high voltage input must not overpower the much smaller logic level voltages attendant to normal functions of the device, such as data handling or supply voltage.